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Solving a Cyclic Symmetry Analysis

Figure 7.13: Process Flow for a Prestressed Modal Cyclic Symmetry Analysis

The modal cyclic symmetry solution occurs after the static cyclic symmetry solution. The modal solution uses the same lowand high-edge components defined in the static cyclic analysis stage (via the CYCLIC command). The analysis yields the eigenvectors of the structure in the prestressed state.

7.3.3.4. Solving a Large-Deflection Prestressed Modal Analysis

Geometric nonlinearity occurs when the deflections are large enough to cause significant changes in

the geometry of the structure. In such cases, the equations of equilibrium must account for the deformed configuration. When a nonlinearity is present, the program uses an iterative process to obtain the solution.

To calculate the frequencies and mode shapes of a deformed structure, you can perform a prestressed modal analysis (using the linear perturbation solution method) of cyclic structures after first performing a large-deflection (NLGEOM,ON) static analysis. Other nonlinearities such as frictional contact may also be included.

The prestress effects are automatically accounted for in the linear perturbation modal analysis procedure.

To obtain the cyclic symmetry modal solutions of a deformed structure, follow these steps:

1.Perform a nonlinear static solution with the prestress load. Use the RESCONTROL command to define the necessary restart files. Non-cyclic loading cannot be applied.

2.Restart the previous static solution from the desired load step and substep.

3.Issue the PERTURB command to define the analysis type, material behavior to be used, contact

status (ContKey = CURRENT, STICKING, or BONDED) and load values to be retained from the previous static solution (LoadControl = ALLKEEP, INERKEEP, PARKEEP, or NOKEEP).

4.Modify the behavior of individual contact pairs, as needed, using the CNKMOD command.

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5.Issue the SOLVE, ELFORM command to regenerate the matrices.

6.Issue the MODOPT and MXPAND commands to specify the modal analysis option.

7.Issue the SOLVE command to perform the eigensolution.

8.Postprocess the results from the Jobname.RSTP file.

The flowchart below illustrates the process involved in a large-deflection prestressed modal cyclic symmetry analysis.

Figure 7.14: Process Flow for a Large-Deflection Prestressed Modal Cyclic Symmetry Analysis

Prestressed Modal

PREP7

Define the basic sector

Perform a large-deflection: NLGEOM,ON

Specify cyclic symmetry:

CYCLIC

SOLUTION

Define static analysis:

ANTYPE, STATIC

Define static loads

Enable file writing for the restart step: RESCONTROL,define

Define cyclic option

CYCOPT

Obtain the prestressed static solution: SOLVE

SOLUTION

Perform a static restart with perturbation: ANTYPE,STATIC,RESTART,,PERTURB

Define the options for the perturbed modal analysis: PERTURB,MODAL

Reform the element matrices:

SOLVE,ELFORM

Define modal options: MODOPT

Define cyclic options: CYCOPT

Solve modal analysis: SOLVE

POST1

Specify the result file for the linear perturbation: FILE,,RSTP

Read the results: SET

Calculate max/min results: CYCPHASE

Display results on full model: /CYCEXPAND

For detailed information about the linear perturbation analysis procedure, see Linear Perturbation Analysis in the Structural Analysis Guide. In addition, Example 9.4: Contact Status Control in a Linear Perturbation Modal Analysis in that same document provides a complete example input listing for a linear perturbation cyclic symmetry modal analysis.

 

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